How can the dew point be a negative value?

Question

What happens if the temperature reaches.. say... a negative 18 degree Farenheit value? Will some crazy inverted ice drops form or is this just some scientific theoretical thing that is suggesting that if that temperature was reached.. liquid would actually form on grass... hmm.. discuss.?

ok, I'll clarify with an example.

During our warmer seasons the Dewpoint can be around 52 degrees Farenheit and when the temp reaches that level.. droplets of liquid dew form on grass..

I the dewpoint is determined as a negative temperature, does 'dew' then form as droplets of ice?

My first posting was a lame attempt at approaching my question in a semi humorous way

Answer 1

To be honest, I don't really get the question, but the dew point is fairly often a negative value (talking in Celsius, that is, but it could as well be negative in Fahrenheit), and it is not uncommon for clouds to be made of small ice crystals.

If the temperature is close to the dew point, the clouds will most likely be low, or there will be fog. Dew point is nothing more than a temperature to which the air must be cooled at constant pressure in order to become saturated with water. Then if condensation nuclei are present, the water will condense. It is not just theoretical, in fat whenever you see clouds it means that the temperature there has reached the dew point. When there is water on the grass (you can especially see this on cool mornings), then yes, the temperature has reached the dew point.

I just read your explanation. I would say yes, if the dew point is negative, and the temperature does cool down to the dew point, then ice crystals will form on the ground. I think that is what English calls frost.

Answer 2

The answers above pretty much sum it up. I want to try to dummy it down a little for you though. Not saying you are a dummy, but I have a fairly good understanding of this and I even had trouble reading some of the answers above.

Dewpoint is a measure of the amount of moisture in the air. If the dew point temp meets the air temperature the air becomes saturated. The dewpoint can never be higher than the temperature. Therefore, the higher the temperature is, the more moisture the air can hold. If you have dewpoints in the 50s as you mentioned there is a LOT of moisture in the air. And, with the slightest bit of instability you can have thunderstorms. Normally you see snow with temps around 32F, plus or minus a few degrees. This is because lower temps don't really carry enough moisture in the air to produce precip. Well, that coupled with the fact that it's hard to get warmer air below the cold air for the lift you need at those temps. If you were to have negative F temps it would be pretty unlikely that you would see any type of moisture, ice or otherwise. But, it is possible to see frost as mentioned above. The little amount of moisture in the air will freeze to surfaces, such as car windows when it reaches saturation, again Temp and Dewpoint are the same. The closer to 32F you are the more moisture there is in the air and the more frost you will see on your windows. But there are other factors that come into play with a frost forecast also. In the warmer months when you see the dew on the grass that you mentioned it is because the dewpoint may be.....oh let's say 45F and the temp 52F in the evening. Then overnight the amount of moisture in the air remains the same, so your dewpoint stays 45F, but the temp cools to 45F also. So saturation is reached and dew forms on surfaces.

Answer 3

Although dew can form when the temperature falls below the dewpoint, the sewpoint itself is the temperature at which the saturation vapour pressure for the air is reached. This could be up in the air as well as on the ground.

If the dewpoint is below 0°C than the water vapour turns directly to ice without going through a liquid phase. This is called deposition and the result on the ground is frost.

Answer 4

If you use the Kelvin scale, the number will always be positive. A negative value is just a quirk of the two scales commonly used.
If the dew point is above the freezing temperature you get water condensation when the temperature drops. (If it continues to drop below freezing you may end up with beads of ice)
If the dew point is below the freezing temperature, the water vapor will 'sublime' and you will get crystal formation. (It could be snow flakes or frost on a surface)

Answer 5

The dew point can be a negative value when the air gets so cold that it actually freezes all of the moisture in the air and not just the stuff you can see. So, it is kind of like a vacuum where the cold just sucks the moisture out of the air by freezing it. Technically, the water is still there, just in a different phase.